1. Field of the Invention
The present invention relates to an image sensor used for a facsimile machine, an optical character reader, etc. and a facsimile apparatus incorporating such an image sensor.
2. Description of the Related Art
A conventional image sensor is generally provided with a transparent cover attached to the upper portion of a frame so as to place an object being photographed thereon, a light emitting portion for projecting light onto the object, an optical system for receiving the light reflected from the object, and a light receiving portion for receiving the light from the optical system. The light emitting portion and the light receiving portion are provided on respective substrates which are fixed to the frame. In most image sensors, the substrate having the light emitting portion is attached to the frame in an inclined manner so as to project light onto the transparent cover obliquely, and the substrate having the light receiving portion is fixed to the lower portion of the frame so as to receive the light reflected from the object perpendicularly downward.
The substrate having the light receiving portion is attached to the frame by one of the following three methods:
(a) The upper surface (the surface on which the light receiving portion is provided) of the substrate is pushed downward by an elastic member which is provided in the frame, and the under surface (the opposite surface to the upper surface) of the substrate is pressed against the lower portion of the frame and fixed thereto. PA1 (b) The substrate is fixed to the frame by screwing it from the under surface of the substrate. That is, the upper surface of the substrate is pressed against the frame. PA1 (c) The under surface of the substrate is fixed to the lower portion of the frame by an adhesive.
In the method (a) of pushing the substrate downward and fixing the under surface of the substrate to the lower portion of the frame, the under surface of the substrate comes into contact with the lower portion of the frame. In other words, since the substrate is fixed to the frame at the under surface, the accuracy of the frame and the uniformity in thickness of the substrate have much influence on the focal length between the light receiving portion and the optical system, thereby making it difficult to adjust the focus on the light receiving portion, which deteriorates the picture quality when it is used for a facsimile machine or the like. In the method (c) of fixing the under surface of the substrate to the lower portion of the frame by an adhesive, since the substrate sticks to the frame, it is impossible to replace the light receiving portion.
In the case (b) of fixing the substrate to the frame by screwing it from the under surface of the substrate, since the substrate is fixed to the frame at the upper surface, although the focal distance between the light receiving portion and the optical system is slightly influenced by the accuracy of the frame, it is not in the least influenced by the thickness of the substrate, so that it is easy to adjust the focus on the light receiving portion. However, since it is necessary to form a threaded hole in the frame and to form a through hole in the substrate, not only does the cost increase but also the substrate must be increased in size in order to prevent the strength of the substrate from being reduced due to the through hole.